1. Field of the Invention
The present invention relates to data transmission systems and more particularly to data transmission systems using surface wave filter devices which encode data prior to transmission and decode data in the receiver to recover the original information.
2. Description of the Prior Art
Systems are well known in the prior art wherein matched filters are used to encode data for transmission and for decoding the data with a similar matched filter to recover the original data. The use of matched filters allows the simultaneous transmission of the data without interference.
It is also well known that acoustic surface wave filters can be used as matched filters in such a system. However, in the prior art a pulse code modulator is used to multiplex the signals received from the acoustic surface wave filter to thereby concentrate the signals onto a single coaxial cable for transmission. A pulse code demodulator at the receiving end transforms the signal and deserializes it for presentation to each of the matched filters in sequence. The prior art does not teach how to use an acoustic surface wave filter device as a multiplexor and demultiplexor.
It is, therefore, an object of this invention to utilize an acoustic surface wave filter device to perform matched filtering (encoding/decoding), in a multiple channel simultaneous communication system.
It is a further object of this invention to provide an acoustic surface wave device which transmits and receives digital information from any combination of data communication channels in a bi-lateral direction, simultaneously, over a single transmission line.
The above objects of the invention are realized by the discovery of a set of codes that have good self-correlation (a signal sequence autocorrelated with its matching filter) and poor cross autocorrelation (a signal sequence autocorrelated with a non-matching filter). Furthermore, the codes are chosen such that the superposition of any combination of codes form a resultant code that causes true autocorrelation in the proper filters and rejection by all other filters.
One embodiment of the invention comprises a matched filter for each input channel deposited upon an appropriate acoustic surface. The acoustic waves set up by each filter from its corresponding channel is multiplexed into a common cable by a single output transducer associated with the acoustic surface which receives the waves and sums the waves into a single output.
The transmitted pulses are inversely transformed to recover the original signals by means of a second acoustic surface wave device which is a mirror image of the first acoustic surface wave device. That is, the second device has a receiver for receiving the signals transmitted on the cable and to thereby set up in the second acoustic surface wave device a signal which is identical to the composite signal set up by the plurality of channels in the first device. This complex signal is demultiplexed by means of matched filters corresponding to each channel which are the mirror image of the matched filters on the input acoustic device.
A clocking channel is further provided in the form of a matched filter on the input and output acoustic device to carry clocking information.
The invention has the advantage that is useful in the area of I/0 bound functions, such as, board-to-board wiring in computer systems wherein there is a limitation, due to compact integrated circuitry, as to the number of output pins that are available.
The invention also has the advantage that it allows encoding and decoding of simultaneously transmitted information.
The invention has the advantage that multiple signals can be transmitted over a single cable with a minimum of sensitivity to noise and inter-channel crosstalk.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings.